Atsuko Sekiguchi

Void formation by thermal stress concentration at twin interfaces in Cu thin films

A void formation mechanism was investigated in an electroplated copper thin film on Ta/SiO2/Si. Microstructural observation after thermal cycling indicated that void formation occurred at intersecting points or terminating corners of annealing twins. The calculated stress distribution was compared with experimental results of the void formation tendency. An excellent correlation was found between void formation sites and stress concentration sites. Electron diffraction analysis revealed that most twin interfaces in Cu thin films are incoherent {322} planes. The stress concentration drives diffusion along incoherent twin interfaces of {322} and leads to void formation at twin interfaces and corners.

Microstructural influences on stress migration in electroplated Cu metallization

Stress migration in advanced Cu interconnects leads to device failure and to poor production throughput. In this work, microstructural effects on stress-migration resistance were investigated in two types of electroplated Cu metallization having a 〈111〉 texture and a random texture. Transmission electron microscopy showed incoherent twins in the 〈111〉 textured films whereas coherent twins in the random textured films. The incoherent twins were found to accompany stress-induced voids because of a weak bonding at twin interfaces. Unlike conventional Al interconnects, a strong 〈111〉 texture should be avoided to minimize stress-migration failure in Cu interconnects.

Evaluation of Interface Adhesion Strength in Cu/(Ta–x% N, Ta/TaN)/SiO2/Si by Nanoscratch Test

The adhesion strength of Cu/(Ta–x% N, Ta/TaN)/SiO2/Si was investigated and the feasibility of the nanoscratch technique was evaluated. Because of the grain orientation dependence of critical load owing to elastic/plastic anisotropy, the interface adhesion measured in the mix-textured films of and showed a wide-range data spread to higher adhesion strength compared with the highly textured films. The adhesive energy derived from the measured critical load by considering the residual stress and the effect of grain orientation by the aid of finite element method calculation showed a reasonable agreement with theoretical work of adhesion. It was revealed that the adhesion strength of Cu/Ta–x% N decreases with increasing nitrogen concentration. This tendency can be associated with increasing number of weak Cu–N bonding along the Cu/barrier interface.

A relationship between film texture and stress-voiding tendency in Copper thin films

The origin of stress voiding in heat-treated Cu thin films was investigated in relation to microstructure. Voids were observed at the intersections of twins with grain boundaries or with other twins. Twin interfaces were accompanied by stress concentration due to the elastic anisotropy. Stress concentration was found to act as a driving force for stress voiding. Twin formation and associated void formation could be avoided by controlling the film texture. Texture transition from (111) to (100) was observed in heat-treated films with increasing the film thickness from 200 nm to 300 nm. The (100) oriented films did not show any voids or hillocks. The excellent stress-migration resistance in the (100) oriented films could be attributed to the absence of twins and by small thermal stresses.

The importance of carbon nanotube wire density, structural uniformity, and purity for fabricating homogeneous carbon nanotube–copper wire composites by copper electrodeposition

We present the influence of density, structural regularity, and purity of carbon nanotube wires (CNTWs) used as Cu electrodeposition templates on fabricating homogeneous high-electrical performance CNT–Cu wires lighter than Cu. We show that low-density CNTWs ( 90 wt %) are essential for making homogeneous CNT–Cu wires. These homogeneous CNT–Cu wires show a continuous Cu matrix with evenly mixed nanotubes of high volume fractions (~45 vol %) throughout the wire-length. Consequently, the composite wires show densities ~5.1 g/cm3 (33% lower than Cu) and electrical conductivities ~6.1 × 104 S/cm (>100 × CNTW conductivity). However, composite wires from templates with higher densities or structural inconsistencies are non-uniform with discontinuous Cu matrices and poor CNT/Cu mixing. These non-uniform CNT–Cu wires show conductivities 2–6 times lower than the homogeneous composite wires.

Phase Formation and Growth Kinetics of an Interface Layer in Ni/SiC

The reaction behavior and growth kinetic of reaction layer were investigated in the Ni contact to n-type 6H-SiC. Annealing was performed at temperature in the range between 800 and 1000 °C for 1 to 240 minutes in Ar atmosphere. The interface reaction of Ni/SiC starts with Ni diffusion into SiC. Ni3Si is initially precipitated and subsequently forms the continuous layer of d-Ni2Si. Kirkendall voids are formed at the reaction front. Carbon is segregated in the interface layer of nickel silicide. The growth rate of the interface layer follows a parabolic law, meaning that the growth rate is controlled by diffusion. The growth occurs in two steps at all examined temperatures: a fast growth is followed by a slow growth. In addition, in the late stage, the growth rate changes dramatically below and above 850°C. The observed growth kinetic can be explained by the difference of Ni diffusivity and the required concentration change for phase transition depending on the phase composition and structure. The d-Ni2Si is formed in the early stage, while the e-Ni3Si2 and q-Ni2Si are formed in the late stage below and above 850°C, respectively.

Finite Element Method Analysis of Nanoscratch Test for the Evaluation of Interface Adhesion Strength in Cu Thin Films on Si Substrate

Mechanical processes of the nanoscratch test are investigated using a finite element analysis of Cu/Ta/SiO2/Si multilayer films. The calculated stress distribution at the moment of delamination suggests that delamination occurs in a small region of approximately 100 nm. The driving force for delamination is the stress concentration due to strain-incompatibility at the Cu/Ta interface resulting from the large plastic deformation in Cu. The degree of stress concentration is found to depend on internal variables, such as plastic deformation, residual stress, and the elastic modulus, and on the magnitude of lateral force.